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7. Recommendations for stock enhancement in large lacustrine waterbodies

7.1 Summary evaluation of current stock enhancement practices

It is quite apparent from the stock enhancement interventions adopted by different countries that they have not had a particularly strong biological basis and that there has been a general lack of critical evaluation. Stock enhancements have rarely been successful, except in the case of enhancement with species capable establishing breeding populations. The majority of successful enhancement interventions are almost entirely confined to medium-sized waterbodies, such as the EaKao Reservoir, Viet Nam. For Viet Nam, a substantial database is available and the reservoir fishery is almost totally dependent on stock enhancement; thus, a comparison of the data presented in Tables 22 and 25 is revealing. It is apparent that in most medium-sized reservoirs (Table 22), the stocked fish account for the great bulk of the catch. This is not the case in the two large reservoirs (Table 25), where the return of stocked fish ranges from 0.2 to 5.13 percent and averages only 4.4 percent.

Table 25. Summary data on economic returns of stocking in two reservoirs in Viet Nam (modified from Nguyen 2001)




Average weight (kg)

Harvest value (x103 VND)1


Cost (x103 VND)




Silver carp

777 500

11 429

14 306



29 334

Bighead carp

702 500

10 326

36 038



162 172

Mud carp

492 500

7 239

2 462



2 462

Grass carp

108 750

1 598




1 359

Cost-Benefit = Total harvest sales/cost of stocking = 5.38


Silver carp

820 800

12 065

73 872



138 346

Bighead carp

471 360

6 928

22 483



49 576

Mud carp

430 080

6 322

39 997



33 597

Cost-Benefit = Total harvest sales/cost of stocking = 7.75

1 Price per kg at harvest in VND: silver carp - 13 670; bighead carp - 15 000; mud carp - 20 000; grass carp - 25 000; 12 500 VND = 1 US$ in 1998.

In almost all the cases reviewed, the number of fish stocked appears to have been rather arbitrary and
not based on any apparent scientific rationale. Similarly, the species combinations used may have been more of a reflection of availability, rather than specific knowledge. In mitigation of this, some stocking activities have sought to fill apparently vacant ecological niches. However, in large waterbodies the trophic relationships are extremely complex, and thus basing a stocking strategy on experiences from aquaculture ponds, in accordance with the polyculture principle in which a species is selected to fill a food niche, may not necessarily be the most suitable approach. This is apparent for a number of reasons:

It is also apparent that for most enhancement strategies there was no attempt to correlate the amount stocked to the potential productivity of the particular waterbody. This ought to be the first step before determining the species composition of the seed stocked. There are now a number of tools available for fishery management to predict the total yield from a waterbody. The development and successful application of the Morpho Edaphic Index (MEI) for inland waterbodies in North America (Ryder 1965) was a starting point for this. Although primarily for natural lakes, MEI has been used for this purpose for tropical waterbodies (Henderson and Welcomme 1974).

Other fish-yield prediction indices have been developed, e.g. chlorophyll-a (Oglesby 1977) and shore-line development (Moreau and De Silva 1991). There has also been one prediction index using geographic information systems based on catchment land-use patterns. These appear to be a significant factor influencing reservoir productivity and hence fish yield (De Silva et al. 2001).

Such predictive models are increasingly used in managing individual fisheries, such as in Sri Lanka, through determining the number of operable crafts in a waterbody, a strategy that is expected to be increasingly adopted by other countries.

7.2 Developing a stock enhancement strategy for large inland waterbodies

Poorly planned attempts to stock large lacustrine waters yield very little rewards, except perhaps for the carnivorous, often naturally recruited, indigenous species already inhabiting these waterbodies, for whom the stocked fish provide an easy meal. Despite this, there is no reason to presuppose that well planned and well executed stock enhancement strategies will not yield positive results, even though such instances are very few and far apart (i.e. the occasional enhancement by species capable of establishing self-reproducing populations). The following sections contain suggestions regarding the process that could be followed in order to develop suitable stock enhancement strategies for large, inland, lacustrine waters in the Asian region.

7.2.1 Does the waterbody require a stock enhancement strategy?

Stock enhancement is not cheap, and a basic prerequisite for developing a strategy is to evaluate critically whether or not the waterbody requires any form of stock enhancement. As part of this evaluation, a number of questions must be addressed:

Having clear answers to these questions will indicate whether the waterbody needs regular stock enhancement.

7.2.2 What form should the enhancement take?

Should it be decided that stock enhancement is indeed warranted, the next decisions relate to the form of enhancement:

There are only a limited number of large waterbodies (>600 ha) in Asia that depend on sustained stocking for their fisheries. Perhaps the main example of this is the reservoirs of Viet Nam. Prior to economic liberalization, stocking programmes in Vietnamese reservoirs were heavily subsidized by the central government, and all fishers and managers were on a monthly wage, irrespective of the fishery income. It has been previously pointed out that Viet Nam, in spite of its rich inland ichthyofauna, does not appear to have suitably adapted lacustrine species that could sustain relatively large and intense artisanal fisheries. The reasons for this are not immediately apparent, however. It may be that inland fishery developments in Viet Nam will have to continue to adopt a stock enhancement strategy, and details of such a strategy will have to be determined through trial and error coupled to sound scientific reasoning.

7.2.3 How to decide what to stock?

This is perhaps the most difficult question to answer, but addressing the following issues could ensure that the stocking activity achieves its objectives:

7.3 Constraints to developing and sustaining stock enhancement strategies for large waterbodies

It quite clear from the analysis of enhancements for the individual countries covered in this review, that one of the main constraints to developing and sustaining stock enhancement initiatives in large waterbodies is the lack of economic viability. This lack of economic viability results from the poor planning of most programmes, including a general failure to use scientific criteria in their design. Even when enhancement initiatives are well designed based on the best scientific knowledge, a major practical constraint to developing sustained stock enhancement programmes is the availability of suitably sized fish for stocking (i.e. >13 cm). While the technical capacity exists in most Asian countries to produce large-sized fish for stocking, the culture area (i.e. aquaculture ponds) required and the costs to produce sufficient numbers for stocking constrains availability. In most countries in Asia, the fish fingerlings that are produced are typically channelled into aquaculture and culture-based fisheries (in smaller waterbodies). An additional issue is that most hatcheries are organized for the production of species used in aquaculture, and these are typically exotic species. The species selected for enhancing large waterbodies are quite often indigenous species that are capable of establishing breeding populations. There is, therefore, a significant gap between what is needed for the enhancement programme and what is available.

Quite probably, this situation is the reason why most enhancement initiatives in large waterbodies have been rather ad hoc and ill planned. They are hampered by inadequate supplies of the right species and in some cases, merely serve as a secondary use for "left-over" seed stock that cannot be used for aquaculture or culture-based fisheries.

Fisheries in large waterbodies in Asia are open access, with a few rare exceptions (e.g. Ayunha Reservoir (3 700 ha) in Phu Yen Province, central Viet Nam). This inevitably means that any stock enhancement strategy has to be undertaken by the relevant state agencies for the public good and cannot be expected to be an income-generating enterprise or even to break even. Examples of state investment in interventions of this nature (i.e. those in which the state does not directly benefit) are increasingly rare in the agricultural sector. Earlier in this review, it was mentioned that state-funded stocking of large waterbodies was in operation in Viet Nam until the economic liberalization of the late 1980s, when the stock enhancement programme had to be suspended. Thailand continues to stock its large waterbodies, with a clear understanding that this is for the general benefit of the open-access fishers who continue to rely on these resources. Where this activity persists, it can be said that the decisions on the quantity of fish stocked and the species used are probably more aligned to the available budget than to the actual requirements for intervention. It has been mentioned that many stock enhancement interventions are not particularly well designed, and that almost all of them are not adequately evaluated. This limits the ability to determine critically whether the stock enhancement activity is actually producing any real benefit or whether it is more of an exhibition of the state supporting poor fishers.

If the goal of the enhancement is to develop a sustainable fishery, then some form of cost-recovery mechanism is required, coupled to some form of fishery management. In most cases, this would be realized as access limitations through licensing or the establishment of a "fishers group". Costs are recovered through realistic license fees or levies imposed on landings, the income being used to support the cost of seed for stocking and the management activities of the fishery. Such systems are rather difficult to implement in large waterbodies, since individual landings are dispersed around the fishery and cannot be effectively taxed. The cost of monitoring fishing boats is also high, and the large area to patrol makes illegal fishing quite possible, especially at night.

There is a more fundamental underlying problem and this is that fishers may be more willing to pay for an enhancement if they believe that it has an impact. The difficulty is in demonstrating that the stock enhancement activity is actually having an impact. Stocking with indigenous species (which then form self-recruiting populations) creates the problem that it is difficult to determine whether the fish captured is one that has been enhanced or whether it is naturally occurring. After the stocked fish become broodstock, this issue becomes irrelevant.

Stocking with exotic species is one way of demonstrating the effect of an enhancement activity. To the authors’ knowledge, this has taken place in at least one country in the region because the state agency undertaking the enhancement wanted the fishermen to be aware that the fish they were catching were the result of the agency’s stock enhancement activities. This is an example where the objective of the enhancement is directed primarily at building a relationship between fishers and state agencies in order to justify licensing and/or levies placed on the fishery.

The large reservoirs of Viet Nam are an example where co-management mechanisms have been established with access restriction and cost recovery mechanisms. Again, in this situation there is still little evidence that the stocking activity is cost effective.

The nursing of fingerlings for release to the fishery by fisher groups or individuals is one method to make fishers more accountable for the costs of stocking. In this case, the nursed fingerlings are paid for by the fisher groups. This, therefore, requires that the fishers using a waterbody are sufficiently organized to be able to levy fees on members to pay for the stocking. The land required for nursing in ponds is considerable, and therefore, the nursing of fingerlings in netcages has been suggested as a viable strategy in large inland waterbodies in Sri Lanka and Viet Nam (details are presented in Section 8.3). Local nursing takes advantage of locally available labour and relies on locally sourced materials. Nursing activities also provide opportunities for womenfolk of fisher households to be involved in the management of husbandry-related activities, for example, in the preparation of feeds, ingredients for which are sourced, at least partially, by fishers from the waterbody itself. There are probably more examples of this type of activity where the fish are not actually released, but are retained and fed in cages until harvested for sale. This cage aquaculture is relatively common on large waterbodies, but requires an investment level beyond the reach of many of the poor fishers who are reliant on the openaccess fisheries of large waterbodies.

In most of Asia, large waterbodies are typically under the purview of several different administrative bodies such as irrigation and agriculture authorities, hydroelectric generation bodies and occasionally, forestry or national park authorities. They are rarely, if ever, under the management of fisheries authorities. The stocking of fingerlings in large waterbodies is rarely coordinated with water release schedules nor are suitable structures installed near sluices to prevent the loss of stocked seed. Lack of coordination between stocking events and water release, especially in the immediate post-stocking period before the seed finds its most suitable location within a waterbody, can often result in loss of stocked seed. In smaller waterbodies, there are conflicts between water use for irrigation and retaining the minimum amount of water necessary to support fish or a fishery. In very small irrigation reservoirs, it is not unusual for almost complete drainage to occur. This can lead to conflicts between fisheries groups and groups that rely on water for dry-season irrigation. Unfortunately, except the study of Jhingran (1992), this issue has not been studied in detail anywhere in the world, and it essentially remains an unknown entity that affects the returns from all stock enhancement programmes. Jhingran (1992) reported a nearly 300 percent increase in fish production in three reservoirs in India as a result of coordination between water release and stocking, and provision of devices to prevent fingerling escape from the spillway.

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